Orbiting robots could assist in the repair and fueling of satellites in space.

However, NASA now has a potential solution for such weakened satellites. The organisation wants to send a robot into orbit in a few years and position it to within grabbing distance of Landsat 7. The robot will catch it and refill it in mid-flight using a mechanical arm.

If the mission is successful, it will be the first time a satellite has been refuelled in space. And this mission is only one of a slew of public and private initiatives aimed at using robots to repair and improve the billions of dollars in satellites in orbit.

These initiatives may eventually lead to better and cheaper satellites that reduce the cost of Internet and cell phone networks, improve weather forecasting, and provide unprecedented views of planetary change and the universe. They may even pave the way for a new era of in-orbit construction, with armies of robots erecting satellites, space stations, and even Mars-bound rockets.

At the moment, there are around 4,852 operational satellites in orbit, which perform critical communications, remote sensing, and other functions. Almost all were built with the understanding that if something went wrong, there was no way to fix it. Most satellites also require fuel to modify their orbits on a regular basis. They may become so much space junk once that's gone, contributing to the already massive stream of material around the earth.

"Imagine you're going to buy a car tomorrow," says Brian Weeden, president of the Consortium for Execution of Rendezvous and Servicing Operations, an industry trade group (CONFERS). "And you have to remember that you'll never be able to add additional gas to it." You'll never be able to replace the oil. You'll never be able to maintain or repair anything. You must also use it over the next ten years. Now, how much do you think that car will cost and how complicated do you think it will be? "We've been doing exactly that with satellites."

Engineers build in redundant systems and load as much fuel as they can into satellites to keep them running for as long as feasible. All of this over-engineering raises the cost of producing and launching satellites, which can reach $500 million for a modern communications satellite.

Almost all space construction and maintenance to date has relied at least in part on astronauts, including Hubble Space Telescope repairs and the International Space Station's construction. However, because sending humans into space is too expensive, the endeavour to design robots to perform the task has accelerated in recent years.

"What we'd really like to have is some method of having a robotic mechanic in space that can fix satellites when they break," says Carl Glen Henshaw, chief of the US Naval Research Laboratory's robotics and machine learning branch.

Henshaw is working on Robotic Servicing of Geosynchronous Satellites, a mission supported by the US Defense Advanced Research Projects Agency (DARPA). It would be the first time a robotic vessel has succeeded to catch a satellite that was not specifically built to dock with it if it succeeds in a demonstration slated for 2024. In the Annual Review of Control, Robotics, and Autonomous Systems, Henshaw and his colleagues looked at some of the issues that come with servicing satellites with space robots.

There are a number of issues like this. Existing satellites lack the markers, known as fiducials, that would allow a robot to visually coordinate itself with a moving satellite. There aren't any fixtures for the robot to grab onto. And the parts of a satellite that do protrude, like as antennae and solar panels, are usually too delicate to handle.

Another issue is the robot's and Earth's time difference. Distance and signal processing cause a communication delay of several seconds between a robot in geosynchronous orbit, some 35,000 kilometres above Earth, and its controllers on Earth. As a result, the robot will have to take care of the most important jobs on its own.

NASA intends to deploy an even more ambitious robot beyond 2025. The OSAM-1 (On-orbit Servicing, Assembly, and Manufacturing) robot would first take care of a complicated refuelling procedure for an existing satellite. Then it would show that it can construct entirely new structures in space.

"We want to show that we can construct these things." "This has never been done before," Robertson says. "You can bring your own stuff or have material supplied to you if you have the ability to assemble things in space." You can also construct considerably larger structures."